Hydrogen production

Rhodobacter sphaeroides 2.4.1(R. sphaeroides 2.4.1) is the representative and most studied bacteria of phototrophic bacteria which can produce H₂ continuously in light. Some external factors of R. sphaeroides 2.4.1 could influence the H₂ yield. These factors are culture medium, pH, temperature, illumination intensity and aerobic/anaerobic condition. All of important, internal factors containing ATP, reducing power, activity of uptake hydrogenase and nitrogenase decide hydrogen production. Photosynthetic system provides enough ATP for nitrogenase, and uptake hydrogenases consume the H₂. [1]

In Photosynthetic system of purple bacteria, both the light driven and respiratory electron transfers serve the sole purpose of generating a proton-motive force across their inner membrane (Fig. 1)。Almost all the useful work derived from absorbed sunlight is delivered to the cell in form of the ATP/ADP-couple in purple bacteria (Fig. 2). [2]

The presence of hydrogenase has been found to be a common feature of the photosynthetic bacteria. In vitro studies show the hydrogenase of photosynthetic bacteria to be capable of both hydrogen production and consumption. However, since hydrogen production is attributed mainly to nitrogenase, hydrogen-producing activity of hydrogenase is negligible (if any). Studies seem to verify this assumption for R.capsulatus at least by showing that the hydrogen producing activity of hydrogenase is less than 10% of the hydrogen consuming activity and that the maximum activity for hydrogenase occurs at conditions favorable for H₂ uptake only. [3]So Hydrogen production is associated mainly or completely with the action of nitrogenase. This enzyme catalyzes hydrogen production in the absence of molecular nitrogen[4]:

However, nitrogenase needs sufficient amount of reducing power and energy in the form of ATP to produce H₂, and the most significant role of photosynthetic system is to generate ATP. So the conversion efficiency of light is a limit to produce H₂.

We have already obtained the protons(Nhv) absorbed by sYFP2, and energy can be transferred into reaction center through FRET to excite bacterial chlorophyll P. Then the electrons are transferred to proton quinone through charge separation.

The electron entering the proton quinone Q causes the quinone to become QH₂. With the Catalysis of Rieske/Cyt b (RB) complexes，translocate protons across the bioenergetic membrane, thus storing a portion of the potential energy from the two electron / two proton oxidation reaction in the electrochemical proton gradient, or proton motive force (pmf) The pmf in turn drives the synthesis of ATP at the FO-F1-ATP synthase [2].

Q/QH₂: the oxidized and reduced forms of the native quinone
C(ox)/C(r): oxidized and reduced downstream electron carriers
H+(P)/H+(n): aqueous protons on the positively and negatively charged sides of the energy transducing membrane.

Many studies shows ATP can be synthetized per four H+ through FO-F1-ATP synthase [2].

ATP synthetized will be provided for nitrogenase to produce H₂, Given that the turnover of nitrogenase is 6.4s-1. [1] It still has potential to use more electrons and ATP to synthetize hydrogen.

We can finally estimate the excess H₂ produced through sYFP2.

1. Hallenbeck P C, Yakunin A F, Gennaro G. Electron Transport as a Limiting Factor in Biological Hydrogen Production[M]// BioHydrogen. Springer US, 1998:99-104.
2. Sener, M. K. & Schulten, K. in The Purple Phototrophic Bacteria (eds Hunter, C. N. et al.) 475-493(Springer, 2009).
3. Koku H, İnci Eroğlu, Gündüz U, et al. Aspects of the metabolism of hydrogen production by Rhodobacter sphaeroides[J]. International Journal of Hydrogen Energy, 2002, 27(11):1315-1329.
4. Frank B. Simpson The hydrogen reactions of nitrogenase.